1. Field of the Invention
The present invention relates to a vehicular two-wheel drive and four-wheel drive switching system.
2. Description of Background Art
Heretofore, a vehicle is known in which two-wheel drive can be switched to four-wheel drive.
FIGS. 4 and 5 show an example of the above-mentioned vehicle. A vehicle 1 is substantially composed of a body frame 3 with an engine 2 mounted in the center. A front wheel 4 and a rear wheel 5 are arranged on both sides of the front and the rear of the body frame 3. A steering handlebar 6 is arranged above the front of the body frame 3 for steering the front wheel 4. A fuel tank 7 is located above the engine 2 and attached to the body frame 3. A seat 8 is attached to the rear of the fuel tank 7.
As shown in FIG. 5, each front wheel 4 is supported by a suspension system 9 that is provided on both sides of the front of the body frame 3 so that each front wheel can be vertically moved. Each rear wheel 5 is supported by a suspension system 10 that is provided on both sides of the rear of the body frame 3 so that each rear wheel can be vertically moved.
A final reduction gear for front wheels 13 and a final reduction gear for rear wheels 14, respectively, connected to the engine 2 via propeller shafts 11 and 12 are provided in the center of the front and the center of the rear, respectively, of the body frame 3. The right and left front wheels 4 and the right and left rear wheels 5 are respectively connected to the final reduction gear for the front wheels 13 and the final reduction gear for the rear wheels 14.
A two-wheel drive and four-wheel drive switching system for switching a rear-wheel drive to a four-wheel drive by disconnecting the motive power to be transmitted to the front wheels 4 or transmitting motive power to the front wheels 4 is provided between the final reduction gear for the front wheels 13 and the propeller shaft 11, for example. In the alternative, the two-wheel drive and four-wheel drive switching system is provided between the rear propeller shaft 12 and the final reduction gear for the rear wheels 14, and the front-wheel drive and the four-wheel drive may be also switched.
The switching of the type of drive is suitably performed by a driver according to the state of the road surface and the type of desired traveling.
For example, a two-wheel drive and four-wheel drive switching system having the structure shown in FIG. 6 is proposed.
The two-wheel drive and four-wheel drive switching system 15 in FIG. 6 is composed of two input shaft components 16a and 16b formed by axially dividing the input shaft 16 of the final reduction gear for the front wheels 13 into two and matched on the same axis and a switching unit 17 for engaging or disengaging thereof.
To describe in detail, a cylindrical positioning protrusion 18 is provided in the center of the end face of the input shaft component 16a located on the side of the final reduction gear for the front wheels 13. A positioning concave portion 19 is provided into which the positioning protrusion 18 is fitted so that the positioning protrusion can be turned. The positioning concave portion 19 is formed in the center of the end face of the input shaft component 16b. Both input shaft components 16a and 16b are positioned on the same axis and are connected so that they can be relatively turned by matching both input shaft components 16a and 16b so that the positioning protrusion 18 and the positioning concave portion 19 fit together.
Splines S1 and S2 are formed on the respective peripheral faces of the respective matched parts of both input shaft components 16a and 16b and the switching unit 17 is provided so that it surrounds the matched parts.
This switching unit 17 includes a switching ring 20 on the inner surface of which a spline is formed, which is fitted to the respective matched parts of both input shaft components 16a and 16b so that the switching ring can be slid. The spline is engaged with or is disengaged from the respective splines S1 and S2. A drive mechanism 21 formed by a solenoid is provided for selectively moving the switching ring 20 to a position in which the spline is engaged with only one input shaft component 16a or a position in which the spline is simultaneously engaged with the splines S1 and S2 of both input shaft components 16a and 16b by sliding the switching ring 20 in the axial direction of both input shaft components 16a and 16b. 
The two-wheel drive and four-wheel drive switching system 15 configured as described above is turned to a two-wheel drive in which only the rear wheels are driven by moving the switching ring 20 in one direction by the drive mechanism 21, engaging it with only one input shaft component 16a and disconnecting the transmission of the driving force to the front wheels 4. The switching system 15 is turned to a four-wheel drive in which driving force is transmitted to the front wheels 4 and the front wheels 4 and the rear wheels 5 are simultaneously driven by engaging the switching ring 20 with the other input shaft component 16b in a state in which the switching ring is engaged with one input shaft component 16a by sliding the switching ring 20 and hereby, engaging both input shaft components 16a and 16b. 
The two-wheel drive/four-wheel drive mode shifting device 15 in such a construction takes the two-wheel drive mode in which only rear wheels are driven by moving the switching ring 20 in one direction by the drive mechanism 21 into engagement only with one of the input shaft components 16a to block distribution of drive power to the front wheel 4, and takes the four-wheel drive mode in which the front wheels 4 and the rear wheels 5 are simultaneously driven by sliding the shift ring 20 into engagement with the other input shaft components 16b while being engaged with one of the input shaft components 16a, whereby both of the input shaft components 16a, 16b are connected and thus drive power is distributed also to the front wheel 4.
However, such a conventional type two-wheel drive and four-wheel drive switching system 15 has the following problems that require improvement.
That is, there is a problem that when there is a difference in the peripheral speed between the front wheel 4 and the rear wheel 5 in switching drive types, engagement between the switching ring 20 and the input shaft component 16b does not function and switching may be disabled.
Even if switching is completed, there is also a problem that the splines collide in switching which results in the generation of noise.
To solve such problems, a mechanism for synchronizing (aligning) the spline of the switching ring 20 and the spline of the input shaft component 16b is required. The structure becomes complex when the two-wheel drive and the four-wheel drive switching system 15 is built. The new mechanism that is required is greatly changed from the existing structure.
In the meantime, there is a problem wherein the propeller shaft of the engine 2 connected to the input shaft component 16b has a different form when the two-wheel drive and four-wheel drive switching system 15 is mounted on a different vehicle from the vehicle 1. The form of the two-wheel drive and four-wheel drive switching system 15 also is required to be changed.